Hollow expandable microspheres containing volatile liquid blowing agents encapsulated therein are beneficially employed as fillers in synthetic resinous castings, as bulking agents in textiles and paper, as thin insulating coatings, as blowing agent for other polymers, and the like. The synthesis of expandable particles is disclosed in a number of patents such as U.S. Pat. Nos. 3,615,972; 4,049,604; 4,016,110; 4,582,756; 5,861,214; 5,155,138; EP 559,254; and PCT Publication No. WO/20465. These publications teach how to synthesize thermoplastic expandable microspheres.
The shells of conventional thermoplastic microspheres are expanded by applying heat, but become softened again and, then, easily broken, when reheated. Conventional expanded thermoplastic polymer shells will also be easily broken when kept at high temperature for an extended amount of time. These characteristics of prior art thermoplastic microspheres substantially limit the applications of thermoexpandable microspheres in the area where closed cell and high mechanical strength are required, such as in making polyurethane and polyisocyanurate rigid foams. The inventors have recognized that it would be desirable to produce thermoexpandable microspheres that can start expansion at a relatively low temperature (e.g., 60° or 70° C.) and have a shell polymer of the microspheres that becomes highly crosslinked at a higher temperature (e.g., 120° or 130° C.) when the microspheres are fully expanded. The crosslinking of the shell polymer is inactive at the onset of the expansion temperature and will be activated at a higher temperature when the microspheres are fully expanded and, then, thermoset the shell of the expended microspheres.
In order to create such desirable microspheres, the inventors have recognized several problems in the conventional manufacture of microspheres themselves.
If the aqueous phase in a hypothetical process used to prepare polymer microspheres is considered, the process looks like a suspension polymerization process, i.e. a water phase will contain a stabilizer, no initiator is present, and an inhibitor is added to the aqueous phase in order to prevent homogenous nucleation from occurring in the water phase, resulting in a broadening of the particle size distribution. The inventors have recognized that stabilization of the growing polymer microspheres can be achieved by the addition of colloidal silica. The colloidal silica stabilizer is just one of several components that are needed to be located at the water/oil interface. A polyester (prepared from a combination of diethanol amine and adipic acid in equimolar proportions) is often recommended as a co-stabilizer when using colloidal silica such as taught by U.S. Pat. Nos. 3,615,972; 4,582,756; and 5,834,526. However, substantial difficulties have been encountered in maintaining the desired quality control of the amine/acid polyester as reported in U.S. Pat. No. 4,016,110. Variation in the properties of the polyester result in batch-to-batch variation of the expandable microspheres prepared using the polyester. For conventional polyester, a pH less than 7 has been recommended to impart good stability to the colloidal silica which exhibited behavior similar to a polyelectrolyte, a polycation in this case. The behavior of the polyelectrolyte at an oppositely charged interface depends on the concentration of the polymer added to the system. At low polyelectrolyte concentrations, bridging occurs between particles, and agglomeration of the colloidal silica particles takes place. The configuration of the polyelectrolyte in the adsorbed layer is dramatically affected by the presence of electrolytes in the aqueous phase. Significant flocculation will be obtained with polyelectrolytes of high molar mass at low polyelectrolyte concentration. Thus, the inventors recognize the molecular weight of the polyelectrolyte and the stability of the molecular weight during the polymerization process are very important. However, a polyester structure in the backbone is very sensitive to hydrolysis due to the low pH, high temperature, and the amount of water. Thus, it would be desirable if there were available an improved co-stabilizer when colloidal silica is used in the preparation of polymer microspheres.
The removal of stabilizer used in the preparation of microspheres are usually difficult as described in U.S. Pat. Nos. 5,155,138 and 5,834,526. The remaining colloidal silica on the microspheres after polymerization can influence the removal of water and the expansion of the microspheres. Co-stabilizer plays an important role in the washing off of the colloidal silica stabilizer. As recognized by the inventors, some applications require that the microspheres contain very low water content. Thus, the inventors have recognized the desirability of using a co-stabilizer that can make it easy to wash off the stabilizer/co-stabilizer after polymerization of the polymer particles, resulting in a more simple process and lower water content in the resulting microspheres.
In one application, the microspheres are used in the manufacture of synthetic foam. Foams and processes for their production are well known in the art. Such foams are typically produced by reacting ingredients such as a polyisocyanate with an isocyanate reactive material such as a polyol in the presence of a blowing agent.
Synthetic foams have many uses and are produced in many forms. Rigid foam insulation panels are used in the construction of buildings. Foam bun stock is used for freezer insulation. Flexible foam is used in the manufacture of automobiles and furniture. Shaped foam products are used for building facades and ornamental effects for both interior and exterior uses.
Foam products are generally highly flammable when made solely out of their basic components. A variety of materials have been used in the past for imparting fire resistance to foams. For example, standard liquid flame retardants such as TRIS (-chloro-2-propyl) phosphate products, commercially available as ANTI-BLAZE 80 from Albright and Wilson and as PCF from Akzo Nobel have been conventionally used to increase the fire resistance of the foam. Such additives can be used to produce Factory Mutual Class 1 rated foam when organic halogenated hydrocarbons, such as 1,1-dichloro-1-fluorethane (HCFC-141b) are used as the primary blowing agent.
Since the use of certain halogenated hydrocarbons may have detrimental environmental effects, it is also desirable to provide foam made with a non-halogenated hydrocarbon as the primary blowing agent. However, similar foams made with non-halogenated hydrocarbons, such as iso-pentane and/or cyclopentane, used as the primary blowing agent fail to produce Factory Mutual Class 1 rated foam. In such cases, the use of expandable graphite as a fire retardant for polymer foams can be used as disclosed in International Publication No. WO 01/72863 A1.
Manufacturing foam with a non-halogenated hydrocarbon, such as iso-pentane, as the primary blowing agent conventionally requires expensive safety measures to be taken to avoid the fire and explain hazzard inherent with storing such blowing agents. Applicants have recognized that the benefits of using non-halogenated hydrocarbons in the manufacture of foam can be realized without the conventional safety hazzards through the uses of the inventive microspheres which encapsulates that highly flammable material.